The present invention relates to an apparatus for securing a sheet material, such as a photosensitive sheet, hereinafter referred to as a sheet, and more particularly, an apparatus for securing a sheet under suction, adaptable for use in combination with a process camera, a step-and-repeat machine, an automatic drafting machine, or the like.
Referring to FIG. 9, which shows a vertical type process camera to which the present invention is to be applied, the reference numeral 1 denotes a sheet holding unit, which includes an upper plate 1a on which a sheet 21 is placed. The upper plate 1a is rotatable around a pivot 22 in the direction of arrow (A), with holding of the sheet 21 thereon by suction. In this way, the sheet 21 is positioned upside down at a focal plane 23, at which the sheet surface is exposed to light from an optical system 24.
The prior art apparatus includes a base plate 110 shown in FIGS. 7 and 8, which includes a plurality of rectangular valley portions 60 and hill portions 70 both produced concentrically of the base plate 110. Each valley portion 60 includes an outlet port 120.
A face plate 30 is placed on the base plate 110 (strictly on the hill portions 70), the face plate having a number of suction pores 40 through which air is admitted in the base plate 110 (strictly in the valley portions 60). The reference numeral 130 denotes a communicating path means through which the air is drawn into a discharge 140 under suction by a sucking means (F), the communicating path means 130 is slidable in the direction of arrows (B) for being adjustable the area of the suction zone in accordance with the size of the sheet 21. In this way the sheet is secured on the face plate 30 regardless of its size.
However, because of the arrangement in which the outlet ports 120 in the base plate 110 are in direct contact with the respective valley portions 60, no substantial resistance acts on the flow of air. If the sheet is too small to cover all the suction pores 40, a corresponding leakage of air is inevitable through the suction pores out of the coverage by the sheet. As a result the negative pressure in the valley portions 60 corresponding to the sheet-free suction pores tends to drop. The negative pressure is defined as pressure, given by the absolute value of the difference between the atmospheric pressure and the pressure in the objective place, and here it is identified within the valley portion 60. A negative pressure results from the suction applied through the suction pores 40 because of the low pressure in the valley portion as compared with the atmospheric pressure. The leakage of air through the suction pores 40 causes a drop in the negative pressure in the communicating path means 130, thereby reducing the effective pressure for securing the sheet on the area of suction zone.
If the negative pressure in the base plate 110 is not sufficient for securing the sheet on the face plate, the sheet is likely to fall off. In order to avoid the falling off of the sheet it is required to use other means for covering the pores out of the coverage by the sheet. In addition, the prior art base plate is heavy because of the solid planar body. The base plate is usually made of cast aluminum, and a heavy base plate causes inconvenience in handling it.